


Submersion

by Mercy_Run



Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell (Undertale), Angst, Ecto-Genitalia (Undertale), Ecto-Penis (Undertale), Ecto-Tongue (Undertale), Ecto-Vagina (Undertale), Ectobiology, F/F, Hurt/Comfort, M/M, Mpreg, Multi, Possession, Racism, Selfcest, Slavery, Smut, Undertale Monsters on the Surface, kustard - Freeform, mermaid au, original lore, slave AU
Language: English
Status: In-Progress
Published: 2019-12-22
Updated: 2019-12-22
Packaged: 2021-02-25 21:35:46
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 3
Words: 4,616
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/21902356
Author URL: https://archiveofourown.org/users/Mercy_Run/pseuds/Mercy_Run
Summary: It was the end.  The Humans have enslaved Monsterkind and there was no way out.  No resets.  No help.  No mercy for any of them.  Sans is desperate to find his brother and escapes, only to tumble headfirst into the ocean.  He is rescued by a mysterious monster called Red and wonders if there is a way left to save them, after all.Just who is Red and his own kind that lives in the water?-Currently ON HIATUS until KR Series is complete!  Thank you for your patience!---------------------------------------------------------------------A fusion of a Mermaid AU and a Slave AU for a unique read.  I hope you enjoy!--------------------------------------------------------------------
Relationships: Sans/Sans (Undertale)
Comments: 19
Kudos: 154





	1. Chapter 1

**Author's Note:**

  * For [silverryu25](https://archiveofourown.org/users/silverryu25/gifts).



Everything hurt but Sans couldn’t focus on anything right now, his breath coming in panicked gasps as the trees surrounding him flew by. He could hear the baying of hounds behind him, close enough to keep him going despite his exhaustion. He couldn’t afford to get caught now. This was the first and last chance he had. If they caught him, they would dust him or the dogs would do the job.

His soul felt like it would break at any moment. His magic was too low for anymore shortcuts and far too low to start up an encounter. Sans would be dead before he could defend himself. The humans had paid a pretty penny to own him but they wouldn’t hesitate to kill him for his trouble.

The metal around his neck was another reminder of just how horribly this time-line turned out. Not only did the humans not give them a chance but the entirety of Monsterkind had been enslaved. It had happened swift and brutal, Frisk screaming in his memory as their family and friends were taken away. Their desperate cries asking why they couldn’t reset. They needed to fix this! They needed to Save everyone!

And time had moved on, a final nail in the coffin. This was their new reality and Sans supposed they shouldn’t have been shocked. Humans hated change and they were afraid. When humans are afraid, they get angry. They were brutal when frightened.

It had been half a year since and the small skeleton had continually rebelled against the humans who purchased him. Sans was a ‘troubled case’ and tossed between owners. He couldn’t remember how many but as time went on, it was harder to fight. More dangerous as well, considering his frail health. Still, he fought for one thing. His brother. Papyrus… He hadn’t seen his face in so very long. Too long. Each change of hands had fueled his need to escape.

To save Papyrus.

Or to die in the attempt.

All the skeleton knew was that he couldn’t keep this up much longer. The hounds cries got closer and Sans realized with dread that they had caught his trail, hungry to end the chase and catch their prey. Any uncertainty that they might spare him vanished. They were going to kill him. He was sure of it. Sans’ pace became a desperate dash, blind and reckless.

He couldn’t stop in time between the trees ending and the ground dropping from below him, his weak limbs jerking in the open air as Sans fell. He was helpless against the impact of water and he was fairly certain he blacked out for a little. 

Sans couldn’t breathe.

The weight of his clothing, the lack of buoyancy in his bones and the metal collar humans had locked around his neck just dragged him deeper underwater, the faint light from the sky gradually being swallowed by darkness.

Everything was fading, his soul becoming heavy and strained as Sans’ teeth parted; choking on sea water.

The small skeleton’s eye-lights fractured and then blinked out, his fading vision going dark.

Drifting deeper….

Was this how it was going to end? 

After all of those resets? After all of their struggles and being enslaved to humankind upon resurfacing?

The monster dazedly reached up towards the break in ocean water to air, bubbles escaping his parted teeth and weakening. His joyful younger sibling flashed across his mind. The first time he had held his baby brother. Papyrus’ first visit from Santa. The expression he had worn when first seeing the sun….

Sans gave up.


	2. Chapter 2

It felt like the weight of the world was pining Sans’ body to the ground when he next become aware. That frightening sensation was made even worse by how very weak he felt. The small monster issued a quiet whine, soul picking up speed as he tried to open his sockets only to find that he couldn't.

Had the humans recaptured him after all? 

Sans couldn't remember much of what had happened after hitting the water but he must have managed to swim somehow… Something brushed against his cheek bone and the skeleton’s soul lurched in fear; teeth parting in a shaky gasp. 

Who..?

“shhhh, I got yer. don't be afraid, surface dweller…”

The voice was rough with an unfamiliar accent in it's tone. Clearly masculine. Sans lingered on the unfamiliar title he had been given, confusion touching his features. 

The stranger chuckled deeply in response.

“ah, yer wouldn't know of us, would yer?” The voice mused before changing into pity, “poor mountain monster, yer kind just had to be compassionate and give those filthy humans an opening to hurt yer…”

The light touch trailed down his face before coming to a stop on the metal of his slave collar.

“that leap yer took would have dusted yer if I hadn't already been watching this coast. this territory belongs to me...and yer owe me a life debt,” the voice murmured as Sans gasped wordlessly, trying to muster enough magic to speak.

Life debts were serious among monsters. It was a more solid claim than even the collar Sans wore. Something that was born of magic and that the skeleton couldn't refuse. The act of saving lives was sacred.

“please-” He gasped and the stranger hummed in question.

“what is it yer begging for?” 

“i...don't care what you do to...me, just let me find my brother and...free him,” Sans gasped out, his voice quiet and faint without any excess magic to ignite his eye-lights.

A heavy pause.

“they have yer brother?” The voice asked.

“papyrus…” Sans sobbed and suddenly that touch on his face returned, cradling it and tilting it urgently. He could feel the stranger's gaze boring into his own.

The other chuckled.

“yer just get more interesting, mountain monster… lemme guess...yer name is sans, isn't it?”

Shock flooded his soul, “...h-how…?”

“heh… i can feel it now that yer so close. that familiar sensation of belonging. i know yer very well, sweetheart. there is no mistaking that. but that is neither here nor there. i know what i want in return for saving yer life.”

Sans’ breath hitched at the sudden hungry tone, shaking helplessly against the hard ground. He was too weak to fight this person, he couldn't do anything if they wanted to hurt him. Blind and weak, Sans was completely at their mercy.

“bond with me, sans.” The voice murmured and then quiet laughter erupted from the other at his expression, “what…? yer thought i was one of those wretched creatures? no… i am a monster...just like you.”

It felt like Sans had missed a joke somehow but he managed to laugh at the irony anyway.

“i’m already... property anyway. i have nothing to lose by changing who has authority over it. sure, i-”

“that is for saving yer. now, for the ability to break this collar off any monster safely..”

What?

“you can...do that?”

Sans’ mind could recall the frightened faces of his little brother, of his friends, of every monster that had the misfortune of being enslaved by the human race. The stranger laughed, the sound echoing oddly in a way that had the skeleton wondering where they were.

“i can, sweetheart. don't worry about that. for that ability to save yer brother and anyone else yer desire…”

There was a pause and the small skeleton monster waited anxiously for his unknown savior to ask what he wanted.

“a child.”

Sans’ mind blanked in utter surprise. The monster couldn't mean..

“...like a human child?” He guessed, assuming that a kid’s soul might aid this stranger. Amused laughter rang in what was unmistakably a cave around them.

“oh, sweetheart. yer a riot. no, i have no interest in a human brat. what i want is yer children. don't try to tell me yer can't 'cause yer male, i know better.”

Sans stuttered in shock, very much certain he would be totally flushed with magic if he had any to spare. The skeleton had never thought about being bonded to anyone, much less create a family with them. The future wasn’t even a remote possibility until Frisk had suddenly lost their ability to reset. Of course he had raised Papyrus but he had never thought beyond his broken world view.

…and with the humans enslaving his race, Sans had thought there would be no future anyway.

“even if i free them, where will they go?” He choked out, “the humans will destroy us…”

“shhh,” the stranger hushed, caressing his face in an overly familiar way that had Sans shivering, “yer don’t have to worry about that right now. magic is fickle but it will provide for it’s children. so… give yerself to me, sans. let me have yer body and yer soul…”

What did Sans have to lose besides his life at this point?

“i….”

He inhaled shakily as the stranger waited for his reply with an unusual amount of patience.

“..i accept.”

A deep humming purr immediately started at his response and Sans trembled as the stranger’s grip moved to his shoulder and collar. A sudden tug had him yelping as air rushed around his weak body and they both hit water. A fearful whine was swallowed by the other monster as he expertly captured Sans’ mouth and kissed him breathless. The transfer of magic was instantaneous and Sans gasped, pulling back as his eye sockets opened sluggishly.

Tiny white eye-lights flickered into existence to see the monster who he was giving the entirety of himself to. The skeleton blinked, shocked at seeing another skeleton slightly bigger than himself staring back at him. The strange monster was treading water, his bones exposed in a way that had Sans’ flushing. He could see the vivid crimson color of the other skeleton’s soul glowing between ribs that looked roughed up. Eye-lights of that same hue were boring into his own. 

Was that…Determination?

The other skeleton’s teeth were sharp, one of them replaced with what looked to be gold replacement. An old injury that made Sans’ own skull hurt ran from that fang up to the socket and up that side of his skull, a severe crack in bone.

This person looked so much like him…

A ruby glow under the water around them drew Sans’ gaze and his breath hitched. A massive tail made of magic curled expertly in the water to draw them both deeper into the darkness of the cave, away from where Sans had been laid to rest on the rock. The other’s movement was sure and strong in the salt water as he smirked at Sans’ shell-shocked expression, sharp teeth tilting upwards in amusement.

“deep breath, mountain monster…”

Unable to wrap his mind around the shocking similarities between them and that he was meeting a human myth, Sans obeyed automatically. He was dragged into the dark water, clinging desperately to the other skeleton as a translucent tail propelled their descent into the depths.


	3. Chapter 3

**Summary for the Chapter:**

> Everything past this is new! ^_^ Thank you so much for reading!

The glow from the skeleton’s long tail illuminated the dark water around them as the stranger pulled Sans back into the dangerous depths. He kept expecting the mer-creature to pull up to swim to the surface but they kept going deeper. Bubbles escaped from Sans’ teeth as his grip tightened on the other, who merely smirked down at his fearful expression. Surely he knew Sans couldn’t hold his breath forever?

They continued down.

Sans’ eye-lights dimmed and fractured as panicked bubbles burst from between his fangs.

The mer’s smirk softened and the stranger let Sans drift to a halt before he was pulled into an embrace. Unable to register much with his magic levels dropping, Sans merely tensed as the other pressed close-

-and captured his mouth.

Sans’ eye sockets widened reflexively and gasped as the mer-skeleton exhaled, the other monster’s red eye-lights growing larger as he pulled back with a smile. Sans’ blinked, realizing the stranger was literally giving him air to breathe and keeping him alive. The mer-creature grinned at him and pulled his smaller body close, scanning his expression before tightening his grip. One flex of the mer’s tail had them speeding through the water. Sans closed his eye sockets at the rush of water and the odd feeling of it moving into his skull. His vision wasn’t good so he surrendered to the fact that he was completely vulnerable.

Their path slowed and the mer swam up to a cavern in a coral reef, the only light down this far coming from the long glowing tail and fins of his future mate. The stranger checked Sans’ expression and smiled patiently, waiting until Sans reached desperately for him before exhaling into his seeking mouth. Sans pulled away, skull flushed with blue magic in being reduced to seek air from the other.

His flush deepened and the mer-creature chuckled before tugging Sans into the cavern surrounded by coral. Immediately, ambient magic lit up the deeper recesses, revealing this to be a living space. Sans spotted some belongings as well as what was unmistakably bones belonging to deceased humans before his body was tugged upwards-

Air?

Sans gasped in deep breaths, eye-lights small and darting around in confusion. There was water below him as well as the mer-creature’s dwelling but this small chamber had a completely dry room. It was small and circular, with a ruby shimmer in both the air and the smoothed rock.

There was a disturbance in the water and Sans flinched back, causing the other skeleton to frown slightly at his reaction.

“did yer think i was going to drown yer, sans?” The mer-creature questioned in his rough voice, appearing genuinely perplexed at his reaction. His eye-lights searched the other’s face intently, blushing.

“i… i wasn’t sure. is this…an air pocket? this far down?” Sans questioned, peering around at their surroundings if only to have an excuse to escape from the other’s intense gaze. The mer smirked in pride.

“it took a lot of magic to maintain this area, but yer are correct. usually i drag prisoners down here…” Sans gaze snapped to the stranger in open horror and worry, “heh… don’t worry, sweetheart. yer aren’t a prisoner.”

“…and you kill them?” Sans had to ask, remembering the remains that were oddly close to them in appearance. 

The mer’s expression darkened, ruby eye-lights darting to the side as he easily circled Sans in the water, “most of them. no children. no innocents. those i returned to the surface. everyone else was left to the water… sans, i know yer have a lot of questions. mountain monsters are so young and curious but i can only grant yer three answers. no more.”

The stranger’s voice was steady and solemn as his eye-lights bore into Sans’. He didn’t doubt the other; though he had to swallow down using one of his three questions to ask why he only got that many. His white eye-lights shrank as he shifted through his curiosity, trying to narrow them down.

“you can really give me the power to get these collars off?” 

The mer tilted his skull and smiled confidently, “yer are so young, sweetheart. i forget how much knowledge yer have lost… what i can do, so can yer through a soul bond. i will share this power willingly for what yer have promised to me.”

What he had promised…

“why…” Sans’ gaze followed the other’s magic weaving through the air, “why do you want to have children with me?”

His smaller body was pulled effortlessly through the water and his eye sockets widened as the other monster caged him in against the side of the rock-face, away from the part of the room he could stand away from the water. The other cupped his face, expression oddly intense.

“it’s a simple matter of clan, sweetheart. yer know the instinct, even away from the ocean as yer were. family… forming ties, fostering strength and size. that is everything to the mer.” The other monster told him.

“and it had to be me? why?”

“…pass…”

“what?” Sans’ eye sockets widened at the firm word, confused.

“ask another question.”

Why wouldn’t he answer this one?

“…fine.” He murmured, wondering what the mer was keeping from him, “…do you have a name?”

The mer’s eye-lights grew larger and he smiled, “yer can call me ‘red’, sweetheart. now… “

Sans’ breath hitched as Red’s hands found his rib-cage and his smaller body was maneuvered away from the wall and towards where Sans could crawl up onto dry surface. He did so easily with the mer-creature aiding him and curled up on the floor with a nervous expression.

He was suddenly frightened. Sans had made all of these promises without much forethought.

The small monster had only promised his body and soul to Red. Offered something as permanent as a child. He knew nothing about this monster that lived in the ocean or how they would raise any children… Would he never see his children? How did this work?

Red purred softly and pulled his face down to nuzzle into the vertebrae at his neck.

“don’t be scared, sweetheart. trust me. i have yer.” 

The other skeleton stayed that way, whispering reassurances until Sans’ shaking subsided before pulling away,”now stay here as i get yer something to help. i will be right back, sans.”

The mer disappeared below the surface without so much as a splash. Sans stared at the still pool of water and his eye-lights shrank. He could almost feel the pressure of the ocean pushing down on him, crushing him… He couldn’t go back now or change his mind. Leaving here would be certain death and staying would…

What had he gotten himself into?

A disturbance on the surface of the pool drew Sans’ immediate attention, the mer resurfacing as Sans fought to keep the claustrophobia at bay. Being down here like this reminded him of Mount Ebott but it was a lot more closed-in. The skeleton shifted towards Red curiously, crawling back within easy reach. His open acceptance had the mer-creature smiling up at him, pleased; before a cloudy glass bottle was held out towards him. 

“what’s this?” He asked, hesitantly taking it. The liquid inside was glowing and had a thick consistency.

“it’ll make this easier for yer, sweetheart,” Sans flushed at the pet name, noting that Red had been calling him that along with the other unusual names, “yer promised me children and yer need to be in heat to do that.”

Oh…

Oh!

Sans’ blush deepened, glowing bright blue as it finally sunk in just what he had been offered. He had gone into heat a few times but the resets had prevented him from reaching that ideal level of happiness and feeling safe enough that his body would do so. What he could remember of those rare times was that his inhibitions were completely destroyed; causing him to lock himself away in his lab until the heat cycle had ran it’s course.

That he was going to be reduced to that vulnerable state somehow hit Sans harder than offering his body and soul to Red, who was a stranger.

“what’s wrong, sans? yer having second thoughts?” The slightly larger monster reached up and cupped his flushed skull, pulling his face down a bit so that their foreheads touched. Sans’ tiny eye-lights trembled as insecurity and nervousness caused his soul to pound underneath his drenched clothing.

“i-i’ve never…” His voice trailed off, unable to complete that sentence.

Red’s eye-lights shrank in surprise before dilating in clear desire, “fuck, sweetheart. yer making it hard for me to be patient…”

Sans shivered at the raw lust being directed at him before glancing down at the glass vial he had been given. Once he stepped over this line, the small skeleton couldn’t go back. He would have a mate and a child on the way without properly getting to know Red at all. That he was bonding to a virtual stranger…didn’t matter in the long run. He was doing this for Papyrus. Who knew what his younger brother was going through right now? 

He had to save Papyrus…

Red watched him with an intense expression as he uncorked the bottle and took a deep breath before knocking back the liquid in one go. He swallowed it before the bitter taste registered, causing him to cough and make a face. The mer chuckled, sharp fangs tilting upwards in amusement. The small skeleton didn’t have to wait very long to feel the effects, his bones warming up at an alarmingly swift rate. He was still shaking as Red pulled him gently down into the water with him, the mer supporting his weight effortlessly. The sensation of the other monster’s glowing tail brushing against his leg had a soft noise of surprise escaping between his teeth.

His rationale was quickly being eaten away by the rising heat in his body, eye-lights hazing slightly as Red shed his clothing; the fabric drifting down into the mer’s dwelling. Still feeling embarrassed when Red’s eye-lights finally took in his completely bared body, Sans buried his face into the monster’s shoulder. The other rumbled an amused chuckle before he was maneuvered, his spine pressing against the rock wall of the chamber.

“hah…”

Everything felt hypersensitive and Red brushed his tongue over his collar-bone, trailing skeleton kisses up his vertebrae. Red grabbed the metal collar around his neck and pulled him close. The mer-creature’s tongue digging into the grooves of his neck had Sans jerking in the other’s grip, gasping as he arched. His legs kicked in the water uselessly before managing to wrap around the base of Red’s glowing tail, drawing a growl from his future mate as his pelvis ground against the ecto-flesh.

“fuck, sans…” Red snarled against his neck before pain bloomed there. The other’s fangs sank into his vertebrae, drawing a surprised cry from Sans as he clung to Red. Cyan tears leaked from his eye sockets as Sans bore the pain of the claiming mark before the mer drew away to soothe it with his tongue. The hurt was swiftly nursed away and the skeleton couldn’t take it any longer; pressing his mouth to Red’s to taste his own marrow and the other’s magic.

Red held him tight, thoroughly plundering his mouth until Sans had to break away, gasping for air as their combined magic dribbled down his chin. The mer’s eye-lights were blown and pulsing hard as he stared hungrily into Sans’ own.

“i’m sorry, sweetheart. i can’t wait any longer…”

One of the other skeleton’s hands reached up into his rib-cage and wrapped around his fragile soul. The feel of Red’s phalanges against the soft inverted shape had pleasure spiking through Sans’ entire being, blue magic curling down his spine to pool at his pelvis as he rutted mindlessly against the mer. His entire focus was on his bared soul as Red smirked and brought it to his sharp fangs.

“red…” He pleaded, his words easing into a sharp cry as the mer-creature opened his mouth, wrapping his tongue around his soul. Red didn’t stop there and actually drew the entire organ into his mouth.

Sans screamed at the dual sensation of his soul being cradled against the other’s flexing tongue and his magic manifesting into a slit to rut against the mer. His hips were caught in a strong grip as Red purred around his soul.

It was too much!

He came without Red even touching his pussy.

Blue magic gushed past the mer’s sharp teeth and he snarled around the soul, swallowing the excess fluid down as Sans fell apart in his arms. He barely felt the brush of something against his folds until pressure on both of his entrances drug him from his delirium. Blinking away overstimulated tears, the smaller skeleton glanced between them and moaned at the sight of two rigid shafts between them, throbbing against his conjured magic from a slit in the base of the mer’s tail.

Intent flooded into him through his trapped soul and Sans keened lowly. Overwhelming desire to see this heat cycle to fruition had him grinding down against the dual shafts, his pleasure intensifying as Red moaned around his dripping soul. The mer-creature brought one hand up and gingerly transfered the fragile organ to his palm, holding it and allowing Sans to calm himself a little.

His respite was brief as the mer’s eye-lights met his own, the hand curled around his soul pressing it to Red’s sternum. In a flash of light, the other’s soul was summoned free and they brushed each other.

Sans’ vision whited out, unsure if it was him screaming or if it was Red. If he had found release or not… Nothing mattered but his soul joining with the stranger’s…

Only…

Red didn’t feel like a stranger anymore.

The skeleton knew that Red could search the entirety of his being and he could do the same but he waited to see what the mer would do first. Shockingly, his memories weren’t raided. There was only an overwhelming sense of belonging. Of a gentle possessiveness. Of…

His eye sockets blinked up at Red as the other murmured affectionate words before their joining was complete, Sans’ soul coming away with a ruby aura around the blue organ. Red’s faded into his ecto-body and the heat was back with the vengeance of a natural disaster, the mer gently returning his soul back to his rib-cage.

Sans moaned and spread his femurs, feeling the other skeleton’s overpowering desire for him and creating a family with him. Logic fled under the emotions and Red was whispering against his skull as he lined both of his dicks up with an entrance each.

“shhh, i got yer… yer so gorgeous, sweetheart. yer with me now, just like yer always belonged. we’re gonna create so many beautiful kids together, sans, just yer wait…” Sans cried out as Red breached his magic, firmly but slowly sinking inside. Forcing his walls apart, though the water and Sans’ previous release probably made it a lot less painful than it should have been for his first time. Red’s twitching shafts worked inside him, twitching and throbbing as the mer cursed and murmured sweet nothings to him.

His mate wanted a child…

Sans keened and jerked his hips down to hilt Red the rest of the way, drawing a startled cry from the mer. One of Red’s hands grabbed the collar and steadied him when the small skeleton splashed in the water, seeking pleasure.

“fuck!”

One powerful buck of the other’ tail had Sans tasting just what his heat demanded and they both fell into a rough, desperate pace. Red yanked him close to swallow his cries as both cocks dragged against his walls on the way out only for the small monster to be delightfully filled once more. His magic barely contained his mate’s girth stretching him impossibly wide.

“red! … red, please! f-faster, more!” Sans cried deliriously, meeting the mer’s thrusts, “f-fill me up, oh s-stars!”

His magic clamped around Red as the mer snarled his name and fucked him right through his orgasm until his mate was spilling into him. Hot magic flooded both of his passages, Red swelling inside him as the mer-creature slammed their magic together with a finality that had Sans blacking out.

Dimly, he felt water surge around their bodies. 

His mouth was claimed and his mate was breathing oxygen into him as his pelvis tilted up, still seeking pleasure as his world narrowed in on Red.

His soul felt warm and he sighed, breathing in Red’s exhale and succumbing.

**Author's Note:**

> Please let me know of any typos and how you are enjoying the story thus far!
> 
> Thank you so much for your support!


End file.
